Population ecology of the endangered fan mussel Pinna nobilis in a marine lake

Abstract

A substantial population of the endangered Mediterranean bivalve Pinna nobilis exists in the marine Lake Vouliagmeni (Korinthiakos Gulf, Greece). The population density of P. nobilis was estimated in the lake with line transect sampling. Individuals of the youngest age class (small) had peak densities in the 1 to 3 m bathymetric zone and their densities were higher in poorly sorted sediments. Older (large) individuals (belonging to all age classes except the first one, 9 to 11 mo) had peak densities in the 11 to 13 m bathymetric zone. No P. nobilis was found deeper than 22 m. The absence of large individuals in shallow waters may partly be explained by illegal fishing. There are several hypotheses proposed to explain the lack of small individuals in deeper areas, but no defini- tive explanation is offered. In Lake Vouliagmeni, P. nobilis densities were high, although marine sea- grass was completely absent. Thus, P. nobilis does not actually require seagrass meadows, as stated by many authors, and it may exist in large numbers in bare soft-sediment areas as well. P. nobilis grew fast, mostly during the first 3 yr of life, and may live beyond 15 yr. By recording the exact location of each P. nobilis individual within 800 m2 transects, as a pair of coordinates, the exact spa- tial distribution was defined and aggregation indices were calculated. P. nobilis had an aggregated dispersion, but no evidence for preferential settlement near adults or previously-settled individuals was found. The aggregated dispersion of P. nobilis probably relates to the patchiness of the local environment. The size of P. nobilis population in Lake Vouliagmeni was estimated to be 8501 ± 4395 (mean ± 1 SD) individuals, of which 4355 ± 3460 belonged to the first age class and 4146 ± 1405 belonged to all other age classes.